The present disclosure involves the qualitative analysis of gases for compounds of interest and is an extension of the apparatus and methods taught in U.S. Pat. No. 5,153,519.
The referenced patent discloses the creation of several charged species by a pulsed direct current (DC) spark discharge acting on a carrier gas containing other compounds to be identified and quantified. The carrier gas is preferably an inert gas and is typically helium. The charged species are used to classify and/or quantify the unknown compounds in the carrier. This detector is connected with upstream or downstream devices such as a sample source, gas chromatograph (GC) column, spectrum analyzer or the like. A sample to be analyzed is loaded for flow along with the carrier gas into a system chamber. While the sample passes through the detection device, a pulsed, high voltage DC spark discharges to form selected charged or energized species within the gas. The spark discharge simultaneously initiates several types of detection systems. For instance, the very short DC spark creates a readily available thermalized electron flux which can be used in a detection system. In an alternate mode of operation, the spark creates a more slowly diffused flux of metastable helium atoms which drift toward selected electrodes within the detector at a controlled rate. The helium atoms will react with molecules of the sample to surrender the excess energy from the excited state to cause sample molecule ionization which, as a secondary and delayed reaction, can be measured by a detection system. Another aspect involves photoionization of gas into positive and negative charged particles normally recombining at high speed. If a select sweep pulse voltage is applied, the recombination is prevented to furnish a signal indicative of the unknown compounds within the gas mixture. Identification and quantification of compounds of interest can, to some extent, be controlled by varying the timing of the spark, the electrode geometry, the voltages of the detector segments, and the modes of interactions observed within the plasma. A complete discussion of the apparatus and basic principles of the measurements are disclosed in detail in U.S. Pat. No. 5,153,519 and are entered herewithin by reference.
There are several limitations to the means and methods disclosed in U.S. Pat. No. 5,153,519. As a first example, the technique provides little control of the compounds within the carrier gas and sample gas mixture which are ionized and therefore detected. If, as an example, it is desired to measure a trace impurity compound in air and both the trace compound and the major constituents of air all are ionized, the relative magnitudes of the major air constituents will introduce serious signal to noise problems thereby degrading the measurement of the desired trace impurity. The teachings disclosed in previously referenced application Ser. No. 08/176,968 present means for not ionizing the major constituents of air, but at the expense of not ionizing, and therefore not detecting, some classes of compounds which may be of interest. In addition, the apparatus disclosed in several of the previously referenced devices are constructed such that the chamber electrodes are exposed to a mixture of carrier and the compound to be detected. Often the compounds of interest are corrosive resulting in corrosion of the discharge electrodes thereby affecting the operation of the measuring system as a function of time. The previously referenced Ser. No. 08/349,039 overcomes some of the afore mentioned shortcomings of the cited reference devices. First, the sample gas is not passed through the discharge electrodes thereby minimizing corrosion of the electrodes. Second, selective ionization of trace compounds within the sample is accomplished by selecting the dopant of the carrier gas. This, however, requires prior knowledge of the major constituents of the sample gas and the ionization potential of the trace compound to be measured within the sample gas. As an example, it might be of interest to monitor commercially produced nitrogen dioxide (NO.sub.2) for a trace impurity such as boron triflouride (BF.sub.3). Helium doped with xenon is an ideal carrier gas to measure in that the ionization potential of BF.sub.3 is below the resonance of xenon while the ionization potential of NO.sub.2 is above the resonance energy of xenon. Any BF.sub.3 impurity is, therefore, selectively ionized while the major constituent NO.sub.2 is not ionized. The targeted impurity can be measured with the signal to noise being maximized using the teachings of the cited application assuming that the afore mentioned characteristics of the sample gas are known. The dopant can not, however, be effectively selected if unless the type of compound to be detected is known.